In turbines, the last stage turbine blade length (aspect ratio) has a significant impact on engine performance. With a longer blade, the speed of the gas stream can be reduced, decreasing flow losses and leading to increased engine efficiency. However, longer high aspect ratio blades suffer vibration problems (e.g. flutter), requiring the addition of means to reduce vibration induced stresses, such as shrouds or snubbers. Such means come with drawbacks such as increased weight or reduced aerodynamic performance. Commonly used under-platform dampers provide insufficient damping for very long blades and can be used only for the vibration modes with insignificant relative movement between neighbouring blades at the platforms. Other means proposed to increase damping include impact dampers (U.S. Pat. No. 6,827,551) and particle dampers (U.S. Pat. No. 6,224,341), whereby a mass or a number of particles are inserted within a cavity in the centre of the blade.
Although impact dampers and particle dampers do provide damping, they also come with several significant problems and limitations. Firstly, considerable design adaptation is required to allow inclusion of these damping means, as cavities must be created within the blade and filled with an appropriate mass or particles. Casting a blade with appropriate cavities may well not be possible, and amending or retrofitting these designs on existing blades is difficult or impossible. In addition, the required cavities may impede the provision of cooling air through blades.
It has therefore been appreciated that it would be desirable to improve the manufacturing process and blade damping design to reduce these problems and limitations.